Z8

Zilog Z8 - eZ8 (current design)

Program counter is 16 bits = 64K with a 16 bit dedicated adder.

Control Registers
  Stack pointer  
    * Earlier versions with smaller register files allowed for external stack.
    8 bit stack only (R255) - used to address a stack implemented within the 
      CPU's register file.   

    Used with R254 (16-bit)  to address a much larger external stack in 
      external r/w program memory.

  Condition (Flags) register
    2 user settable and testable flags - not affected by the ALU or reset.
      Not usable by branch commands.

    Carry flag - notes carry out of or into most sig. bit of register.
    Zero flag - notes if action results in zero.
    Sign flag - most sig. bit of register (sign). 
    Overflow - carry from 7th to 8th bit (assumes value signed). 

    Half carry - used for BCD math.
    Decimal Adjust - indicates if last command was add or subtract.
      Used by decimal adjust command to correct result if using BCD.

  Register Pointer - eZ8 treats the Register file as a cache of registers.
    4 low bits point to 1 of 16 registers.
    4 high bits point to 1 of 9 to 16 register groups. (depends on model)
      Allows for fast context switching between multiple task.
 
    Instructions use a 4 bit address which is concatenated with register
      pointer.  

    Or as a single range of 236 general registers via an 8-bit address field.

 Addressing modes

    * Register
    * Indirect register
    * Immediate
    * Direct address
    * Indexed with 8-bit displacement  e.g  R1,24
    * Program-counter relative

   Register, Indirect register, and Immediate work with 
     Load, Arithmetic, Logical, Shift, Rotate and Stack instructions. 

   Conditional jumps work with
     Direct address and program-counter relative.

   JUMP and CALL work with 
     direct address and indirect register addressing modes.